In general, in a state where a density of fuel differs, a lubricity of fuel and a combustion property of fuel differ correspondingly. Therefore, it may be demanded to detect a density of fuel and to perform an engine control according to the detected density of fuel. For example, Patent Document 1 may disclose that a cycle of a pressure pulsation, which is caused in fuel pressure when a fuel injection valve injects fuel, changes correspondingly to change in a density of fuel. For example, a pressure sensor device may be employed to detect change in a pressure pulsation. In this case, a physical quantity, which includes a frequency of a pressure pulsation or a cycle relevant to the frequency, may be detected as an actual characteristic according to a sensor signal from the pressure sensor device. The present way may enable detection of a density of fuel without using a density sensor.